Changes in orthostatic tolerance in man at an altitude of 3500 meters.

Orthostatic tolerance was measured in 20 lowlander Indian soldiers (sojourners) by recording responses of heart rate (HR), blood pressure (BP) and mean skin temperature (Tsk) to 70 degrees head-up passive tilt, initially at Delhi (260 m altitude) and thereafter at 3500 m at weekly intervals for 3 weeks. For comparison, observations were also made once on 10 acclimatized lowlanders (AL) and 10 high-altitude natives (HAN) at the same altitude. Among sojourners, the percentage of subjects showing orthostatic intolerance (OI) during tilt increased at high altitude in the first and second weeks; six subjects fainted in the first week as compared to one who fainted at sea level (SL). There was no incidence of fainting among AL or HAN and the percentage of subjects showing OI was very small. The magnitude of cardio-acceleration to tilt was higher in sojourners at high altitude (HA) and relatively less in AL and HAN. The systolic BP during tilt showed no change at SL or during the first and second weeks of stay at altitude in sojourners, but in third week showed a mild rise and was comparable to the responses of AL and HAN. The diastolic BP during tilt always showed a rise, although it was of a higher magnitude during the second and third weeks in sojourners as well as in AL and HAN. There was a fall in Tsk during tilt, which was of a lesser magnitude at HA. It has been concluded that there is a reduced orthostatic tolerance at higher altitude during the first week, due to hypocapnia, after which it is improved as a result of relative sympathetic hyperactivity and adaptation of the vasomotor centre to reduced PaCO2 level.